Dark Skin Wash vs Mortarion Grime

How close are they?

ΔE 33.2, loose: different enough that you should pick on purpose. Mortarion Grime is noticeably lighter, more saturated, cooler than Dark Skin Wash.

Can I use Mortarion Grime instead of Dark Skin Wash?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 33). Mortarion Grime is noticeably lighter, more saturated, cooler than Dark Skin Wash.

Can I use Dark Skin Wash instead of Mortarion Grime?

The same difference applies the other way round: Dark Skin Wash is noticeably darker, more muted, warmer.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.